Files: play video and audio in the viewer, as the file is; reads answer Range requests

The viewer hands a video or audio file to the browser untouched — container, codec and
all — and the browser plays what it can; when it can't, the viewer says so and offers the
download. What the server adds is the part a player needs: /api/files/read now goes
through http.ServeContent for a real file of known size, so Range requests (seeking) and
HEAD work, and a download can resume. The root backend's helper takes an offset for that,
and a Seeker opens the backend once, at the range asked for, instead of streaming from
the top. Audio files are a preview kind too: their cover art is the thumbnail.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

internal/api/files_test.go

@@ -0,0 +1,62 @@
+package api
+
+import (
+	"net/http/httptest"
+	"net/url"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"hamlab/internal/files"
+)
+
+// A file read answers Range requests — what a <video> needs to seek — and HEAD, through
+// the same handler that streams everything else.
+func TestFilesReadRanges(t *testing.T) {
+	dir := t.TempDir()
+	p := filepath.Join(dir, "clip.mp4")
+	data := make([]byte, 10000)
+	for i := range data {
+		data[i] = byte(i % 251)
+	}
+	if err := os.WriteFile(p, data, 0o644); err != nil {
+		t.Fatal(err)
+	}
+	s := &Server{fsLocal: files.Local{}}
+	get := func(method, extra string, hdr map[string]string) *httptest.ResponseRecorder {
+		r := httptest.NewRequest(method, "/api/files/read?path="+url.QueryEscape(p)+extra, nil)
+		for k, v := range hdr {
+			r.Header.Set(k, v)
+		}
+		w := httptest.NewRecorder()
+		s.filesRead(w, r)
+		return w
+	}
+	w := get("GET", "", nil)
+	if w.Code != 200 || w.Body.Len() != 10000 || w.Header().Get("Content-Type") != "video/mp4" || w.Header().Get("Accept-Ranges") != "bytes" {
+		t.Fatalf("whole: %d %d %q %q", w.Code, w.Body.Len(), w.Header().Get("Content-Type"), w.Header().Get("Accept-Ranges"))
+	}
+	w = get("GET", "", map[string]string{"Range": "bytes=100-199"})
+	if w.Code != 206 || w.Header().Get("Content-Range") != "bytes 100-199/10000" || w.Body.Len() != 100 || w.Body.Bytes()[0] != data[100] {
+		t.Fatalf("range: %d %q %d", w.Code, w.Header().Get("Content-Range"), w.Body.Len())
+	}
+	w = get("GET", "", map[string]string{"Range": "bytes=9990-"})
+	if w.Code != 206 || w.Body.Len() != 10 || w.Body.Bytes()[9] != data[9999] {
+		t.Fatalf("tail range: %d %d", w.Code, w.Body.Len())
+	}
+	w = get("HEAD", "", nil)
+	if w.Code != 200 || w.Body.Len() != 0 || w.Header().Get("Content-Length") != "10000" {
+		t.Fatalf("head: %d %d %q", w.Code, w.Body.Len(), w.Header().Get("Content-Length"))
+	}
+	if w := get("GET", "&max=50", nil); w.Code != 413 {
+		t.Fatalf("the editor's cap: %d", w.Code)
+	}
+	if w := get("GET", "", map[string]string{"Range": "bytes=20000-"}); w.Code != 416 {
+		t.Fatalf("a range past the end: %d", w.Code)
+	}
+	if err := os.Chmod(p, 0); err == nil && os.Geteuid() != 0 {
+		if w := get("GET", "", nil); w.Code != 403 {
+			t.Fatalf("unreadable: %d", w.Code)
+		}
+	}
+}